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Extreme winds have emerged as a potentially greater risk to the Auckland Harbour Bridge than previously thought - just as Transit New Zealand seeks to quell fears that the vital structure's clip-on lanes could fail.
Among documents released by Transit to back its assertions of the bridge's safety is a discussion of the possible inaccuracy of certain calculations on the effects of extreme winds on the bridge.
Engineering consultancy Beca In-frastructure told Transit in a June report that even on the existing benchmarks, some of the structures supporting the clip-ons "were found not to satisfy standards in maximum winds" when added to maximum traffic loadings.
But these standards in Transit's Bridge Manual did not take account of wind turbulence created by the bridge itself. Wind effects could be 15 per cent greater than previously thought and the new approach is now being tested in a Melbourne wind-tunnel.
Transit had already been battling the revelation that a December 2006 Beca report it released last week had said the clip-ons were at risk of "catastrophic failure" if heavy trucks were forced to a crawl or halt in both of a clip-on's lanes.
What did the Beca report say?
"If the level of load in this assessment representing a traffic jam situation with maximum heavy vehicle concentrations at the centre of span two [the central span] in both lanes were to occur, there would be a risk of catastrophic failure of the deck."
The report was written to assess the clip-ons for Transit. It concluded that in several areas they "fall short of the adopted assessment standards by a large margin".
Is the Beca scenario feasible?
Transit explains it was a worst-case scenario, of a "very large number of heavy trucks stopped across the central spans" by a traffic jam further up the motorway.
Transit concluded there was no real prospect of a such a tail-back, but has restricted trucks anyway.
What safety and strengthening measures are being taken?
In July, Transit banned container trucks and all others weighing 13 tonnes or more from the outer of the two lanes on each clip-on, to reduce fatigue stress. It had earlier asked truck companies voluntarily to avoid those lanes.
Live camera surveillance of the bridge night and day allows diversion of traffic if necessary.
Transit is spending $45 million on strengthening the clip-ons, which were added to the bridge in 1969,10 years after its opening.
Is the truck ban enough?
Transit released a letter it received from Beca last month, to show Beca's approval of its moves. The letter says Beca recommendations last December included that the outside lanes be closed to heavy trucks and a system be introduced to prevent queuing of northbound heavy trucks in the early morning, when their percentage of all northbound traffic is the highest.
The truck ban and "incident control" system "satisfies [Beca's] recommendation", the letter concludes.
An earlier Beca report raising concerns about the clip-ons, from July last year, says traffic loadings may have increased by 10 per cent in the preceding four years.
"Until further assessment and strengthening are carried out we consider it would be prudent to reduce traffic loads, at least to the levels that the bridge has carried in the past."
* Transit's chief executive Rick van Barneveld last night responded to additional questions stemming from the Beca report.
What number of 13-tonne trucks in either of the outside clip-on lanes theoretically produces the risk of "catastrophic failure" of that lane, as hypothesised by the Beca report?
Thirteen-tonne trucks do not present risks that impact on the safety of the bridge. The hypothetical loading would be based on trucks of the maximum legal weight of 44 tonnes.
Does the same theoretical risk arise from buses stopped in those lanes (or is there something different about buses - weight per length for instance - compared with laden trucks). If so, how many buses?
The same argument applies to buses. An average bus weighs approximately 20 tonnes, which is less than the legal weight of 44 tonnes. Furthermore, at the times of day when bus traffic is heaviest, higher volumes of light vehicles/cars will also be present in the traffic mix. This will dilute the effect of the heavier buses. This situation is far removed from the hypothetical loadings used in the analysis.
Why are buses not banned from the outside clip-on lanes?
As described above, buses do not present a risk that impacts on bridge safety.
Are any special measures being considered for the extra buses that will cross the bridge from next year?
For the reasons outlined above, no special measures will be required when extra buses are in use on the bridge from next year.
How serious is the wind threat discussed in the June 2007 Beca report and now the subject of wind-tunnel testing?
We have previously carried out wind loading upgrades to strengthen the Auckland Harbour Bridge as part of our ongoing maintenance and management of the structure.
In one of its recent reports, Beca recommended further investigation of wind loadings for specific parts of the bridge to confirm design assumptions. Wind tunnel testing is underway.
If this testing shows further opportunities for strengthening the bridge, taking advantage of the latest technological advancements, it would make sense to include this work at the same time as the already scheduled strengthening work for the clip-ons.
Transit continuously monitors wind speed on the bridge and there are procedures in place to limit the use of the bridge by heavy/high-sided vehicles through traffic management.
Is heavy traffic already restricted on the clip-on lanes in heavy winds? If so, when were these restrictions introduced? What are "heavy winds"?
Transit continuously monitors wind speed on the bridge and there are procedures in place to limit the use of the bridge by heavy/high-sided vehicles through active traffic management.
Can the clip-ons not be strengthened any more than they are being in the current upgrade?
What are the future implications of this in terms of traffic volumes on the clip-ons and future preventative maintenance and restorative maintenance/repair of them? We expect the current upgrade will accommodate projected traffic growth on the Auckland Harbour Bridge for at least the next 20-30 years.
The need for further upgrading beyond this point will have to be considered in later years and will be influenced by the future development of Auckland's transport network.
